The present invention relates to a current determination circuit which detects and determines a current supplied from a power supply device when the power supply device, which converts a voltage by a switching method using, e.g., a semiconductor element, is used by connecting the power supply device to a load such as an electrical device, and an image printing apparatus using a power supply device which incorporates the current determination circuit.
A power supply device which is used in many electrical devices and supplies power preferably detects a load current supplied from the power supply device and determines the load state. This is achieved by, e.g., an overcurrent protection circuit incorporated in a widely used power supply device.
A general method of detecting a supplied load current is a method using a xe2x80x9ccurrent detection resistorxe2x80x9d in which the load current is detected as a voltage drop by a detection resistance component.
In the method using the xe2x80x9ccurrent detection resistorxe2x80x9d, a larger load current increases the loss by Joule heat generated by the xe2x80x9ccurrent detection resistorxe2x80x9d. Various methods which solve this problem have been disclosed.
For example, Japanese Patent Laid-Open No. 6-201738 (xe2x80x9cCurrent Detection Circuitxe2x80x9d) uses xe2x80x9cthe ON resistance (Rdon) of a switching MOS transistorxe2x80x9d through which the load current flows, instead of the xe2x80x9ccurrent detection resistorxe2x80x9d. The characteristic of xe2x80x9cthe ON resistance of the switching MOS transistorxe2x80x9d is influenced by variations between switching MOS transistor lots of the same manufacturer, and also readily influenced by the use environment (e.g., temperature).
To employ a method of detecting a load current supplied from the power supply device by using xe2x80x9cthe ON resistance of the switching MOS transistorxe2x80x9d, switching MOS transistors with the same characteristic are desirably stably supplied. For this purpose, it is desirable to freely use switching MOS transistors of not one manufacturer but a plurality of manufacturers.
For the switching MOS transistor having the above-mentioned characteristic, a substitute of a different manufacturer fails to obtain the same characteristic. It is therefore difficult in terms of practical use to adopt the load current detection method using xe2x80x9cthe ON resistance of the switching MOS transistorxe2x80x9d under this restriction.
Another method which does not use any resistance component such as the xe2x80x9ccurrent detection resistorxe2x80x9d is a method of xe2x80x9cdetecting a load current as a magnetic component and converting the load current into a physical amountxe2x80x9d, i.e., a method of converting a current into a voltage amount via a magnetic component. As an example of this method, a current detection coil called a xe2x80x9ccurrent transformerxe2x80x9d is used, or a current flowing through a reactor is detected by a Hall element as a magnetic sensor. This method is disclosed in Japanese Patent Laid-Open No. 5-64425 (xe2x80x9cOvercurrent Detection Circuit of Step-Up Converterxe2x80x9d).
The method of xe2x80x9cdetecting a load current as a magnetic component and converting the load current into a physical amountxe2x80x9d can advantageously decrease the loss caused by Joule heat in comparison with the use of the xe2x80x9ccurrent detection resistorxe2x80x9d, and increase the current detection precision.
However, the current detection coil and Hall element are expensive and must be mounted at a location where they can detect a magnetic component, resulting in low usability.
As still another method, Japanese Patent Laid-Open No. 5-292742 (xe2x80x9cSwitching Power Supply Devicexe2x80x9d) discloses a current detection circuit which is electrically connected to the two terminals of a flywheel diode and constituted by a voltage-dividing resistor, filter capacitor, and voltage amplifier.
According to this method, a current flowing through the flywheel diode in proportion to the load current is extracted in mounting. An unwanted loop antenna may be formed depending on the wiring pattern of a printed circuit board, which poses a serious problem in a power supply which should suppress generation of power noise.
As described in xe2x80x9cBACKGROUND OF THE INVENTIONxe2x80x9d, the method of detecting a load current supplied from a power supply device which is used in an electrical device and supplies power suffers problems: (1) the precision is high, but the detection loss is large; (2) the precision is high, but the cost is high; and (3) noise may be generated.
For this reason, the above-described load current detection method cannot be applied to an energy-saving electrical device such as a recently popular printer (e.g., a large-format printer).
When the current is required to be detected in an electrical device, the absolute precision of a continuous current value is not often measured. In many cases, it is sufficient to detect a current value in a specific load state and determine the load state.
For example, the load current used in an overcurrent protection circuit is detected to detect a current in a load state and determine (decide) the operation start time of the overcurrent protection circuit in order to determine the start point of the overcurrent protect operation. In this case, it suffices to determine whether the load current exceeds a preset current value, and the load current need not be continuously measured at a high precision.
The present invention has been made to overcome the conventional drawbacks, and has as its object to provide a current determination circuit capable of determining that the current falls within a proper range while preventing an increase in cost in current detection and reducing Joule loss caused by an unwanted resistance component without employing any method such as a method using a current detection resistance, a method using the ON resistance of a switching MOS transistor, or a current detection method using a current detection coil or Hall element when a power supply device which converts a voltage by a switching method using a semiconductor element is connected to a load (electrical device) and it is determined whether a current supplied from the power supply device to the load falls within a proper range.
It is another object of the present invention to provide an image printing apparatus which uses a power supply device having the current determination circuit as a power supply device for supplying power in the image printing apparatus, and even if a printhead error occurs in printing, can detect the error before printing and prevent wasteful printing suffering from the printing error.
To achieve the above objects, a current determination circuit according to an aspect of the present invention has the following arrangement. That is, a current determination circuit which determines a current value supplied to a load from a power supply device having a power supply circuit which converts an input voltage by turning on/off switching means comprises detection means for detecting a predetermined signal on the basis of an ON/OFF signal used to turn on/off the switching means, and current determination means for determining based on the predetermined signal, of a plurality of ranges, a range within which the current value supplied from the power supply device to the load falls.
For example, the plurality of ranges are preferably two ranges including a range in which the current value is not less than a predetermined current value, and a range in which the current value is smaller than the predetermined current value.
For example, the detection means preferably DC-detects the ON/OFF signal to detect a voltage level corresponding to the ON/OFF signal.
For example, the current determination means preferably compares the voltage level with a predetermined voltage level set in advance, and when the voltage level exceeds the predetermined voltage level, determines that the current value supplied from the power supply device to the load is not less than the predetermined value.
For example, the detection means preferably detects a time width corresponding to an ON time of the ON/OFF signal.
For example, the current determination means preferably compares the time width with a predetermined time width set in advance, and when the time width exceeds the predetermined time width, determines that the current value supplied from the power supply device to the load is not less than the predetermined value.
For example, the power supply device preferably includes a DC/DC converter which steps up or down the input voltage.
For example, the current determination circuit preferably further comprises discontinuous mode determination means for determining based on the ON/OFF signal whether a mode of a current flowing through an inductor of the DC/DC converter is a discontinuous mode in which all energy accumulated in the inductor during an ON signal period of the ON/OFF signal is discharged during an OFF signal period, or a continuous mode in which not all energy accumulated in the inductor during the ON signal period of the ON/OFF signal is discharged during the OFF signal period but energy is accumulated in the inductor.
For example, the discontinuous mode determination means preferably determines, as the discontinuous mode, a case in which an oscillation waveform is generated during the OFF signal period of the ON/OFF signal.
To achieve the above objects, an image printing apparatus according to another aspect of the present invention has the following arrangement. That is, an image printing apparatus comprises a power supply device having a power supply circuit which converts an input voltage by turning on/off switching means, and a current determination circuit which determines a current value supplied from the power supply device to a load, the current determination circuit having detection means for detecting a predetermined signal on the basis of an ON/OFF signal used to turn on/off the switching means, and current determination means for determining based on the predetermined signal, of a plurality of ranges, a range within which the current value supplied from the power supply device to the load falls.
For example, a timing at which a current flowing from the power supply device to the image printing apparatus is determined using the current determination circuit preferably includes a timing when the image printing apparatus does not print an image.
For example, the image printing apparatus preferably further comprises an ink-jet printhead which discharges ink to print information.
For example, it is preferable that the printhead include a printhead which discharges ink by using heat energy, and comprise a heat energy converter for generating heat energy to be applied to ink.
Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings, in which like reference characters designate the same or similar parts throughout the figures thereof.